The receptor site for dopamine: Diazoesters of phenothiazines and of Haloperidol have been synthesized that bind tightly to the dopamine receptor site. Experiments to mark this site by photoaffinity labeling can therefore be begun. Two problems: (a) Phenothiazines themselves (but not Haloperidol) are photolabile, (b) Since the receptor sites are present only in minute concentration, they can be effectively marked only with reagents of exceptionally high specific radioactivity. Photoaffinity labeling: Reagents have now been invented that yield diazo thioesters that in turn undergo photolysis with insertion into surrounding molecules. The new reagents will be allowed to react with thio enzymes (creatine kinase, glyceraldehyde-3-phosphate dehydrogenase, etc.); photolysis should mark residues near the active sites. Orotidine-5'-phosphate decarboxylase: Effective inhibitors (such as hydroxamic acid derivatives of orotidine-phosphate) have been discovered recently. These will be attached to Sephadex by long arms, in an attempt to prepare an affinity column for the purification of the enzyme. Further, certain barbituric acid derivatives have proved irreversible inhibitors for the enzyme. Investigation with labeled barbiturates could help identify an essential residue at the active site.